Checkpoint blockade, which targets inhibitory receptors that suppress successful T cell responses to tumors, demonstrates the critical role that the immune response plays against cancer. However, not all cancers respond to immunotherapy or checkpoint blockade targeting PD-1 or CTLA-4 on T cells. For example, the majority of patients with colon cancer do not respond to checkpoint inhibitor monotherapy, which is effective in only 4% of patients with microsatellite instability. Thus, many tumors may target other inhibitory receptors or different components of the immune system to block it from mounting an effective response. One family of inhibitory receptors on immune cells is the Siglec family, which recognize ligands with sialic acid modifications on cell surface glycoproteins or glycolipids. There are twenty eukaryotic sialic acid transferases, each of which differ in their preferred linkage, targets and number of sialic acids added. It has been long recognized that tumors alter the type and frequency of glycosylation and sialic acid on the cell surface. In particular, increased incorporation of sialic acid is frequently observed in cancer, although the function of hyper-sialylation in promoting and sustaining tumor growth is not well understood. Interestingly, using the COSMIC database, we found that while 389 human tumors over-expressed the sialic acid transferase ST8Sia6, none under-expressed ST8Sia6; this indicates a strong selective advantage for tumors with high ST8Sia6, although the function of ST8Sia6 in cancer is not known. We demonstrated that ST8Sia6 overexpression in either MC38 or B16-F10 tumor cell lines accelerated tumor growth in mice. Tumor associated macrophages (TAMs) in ST8Sia6- expressing tumors possessed an M2-like phenotype, as compared to a M1-like phenotype in tumors formed from the parental MC38 or B16-F10 tumor cell lines that lack ST8Sia6 expression. We demonstrated that ST8Sia6 generates ligands for the inhibitory Siglec, Siglec-E, whose expression is restricted to innate immune cells. The growth advantage of ST8Sia6-expressing tumor cells was lost when injected into Siglec-E knockout mice, demonstrating that the primary effect of ST8Sia6 overexpression is on inhibition of the immune response of the host rather than an intrinsic effect on cell growth. We have developed a novel cre-dependent, dox- regulatable ST8Sia6 transgene to study the effect of ST8Sia6 in spontaneous tumor models. Overexpression of ST8Sia6 in a spontaneous model of colon cancer dramatically decreased survival to 2-3 months instead of 6 months, demonstrating the strong effect ST8Sia6 overexpression has on tumor cell growth in vivo. This proposal will examine the function of ST8Sia6 in modulating the immune response to tumors.